1. Field of the Invention
The present invention relates to a technique of automatic focus control of an image pick-up apparatus such as a video tape recorder integrated with a camera (hereafter referred to as a "camcorder"), and more particularly, to a technique for preventing an incorrect operation in the automatic focus control.
2. Description of the Related Art
FIG. 11 illustrates an example of an automatic focus control apparatus of an image pick-up apparatus such as a camcorder. In FIG. 11, light from an object (not shown) is focused by a focusing lens 1 onto a CCD image sensing device 3 so that an image of the object is formed on the CCD image sensing device 3. The amount of light falling onto CCD image sensing device 3 is adjusted by an iris 2 to a proper value. The CCD image sensing device 3 converts the optical image into a video signal which is then transmitted to a sample-and-hold and AGC circuit 4. In the sample-and-hold and AGC circuit 4, noise is removed from the video signal and the signal level of the video signal is adjusted to a proper value. The video signal is then converted by an analog-to-digital converter 5 into a digital signal. The video signal converted in the digital form is subjected to camera signal processing such as Y/C separation, gamma correction, etc., in a camera signal processing circuit 6. The output signal of the camera signal processing circuit 6 is transmitted to a recording/reproducing circuit (not shown) and is subjected to recording/reproducing processing. The camera signal processing circuit 6 extracts a luminance signal from the video signal and transmits the resultant luminance signal to an automatic focus detection circuit 7.
As shown in FIG. 12, the automatic focus detection circuit 7 includes: a high-pass filter 71 for passing high-frequency components of the luminance signal (or a band-pass filter); a rectifying circuit 72 for rectifying the output of the filter 71; a gate circuit 73 for extracting a luminance signal within a distance measurement frame from the output of the rectifying circuit 72 wherein the extracted luminance signal is used in the automatic focusing operation; and a detection circuit 74 for detecting a maximum value of the high-frequency components within a field from the output of the gate circuit 73 thereby generating a focusing signal.
FIG. 13 illustrates a typical focusing signal generated by the automatic focus detection circuit 7 shown in FIG. 12 as a function of the focusing position. As shown in FIG. 13, the level of the focusing signal has a peak value at the best focus position.
Referring again to FIG. 11, the focusing signal generated by the automatic focus detection circuit 7 is transmitted to a control microcomputer 8. The control microcomputer 8 transmits a motor control signal to a motor driving circuit 9 so that the focusing lens 1 is moved in a direction which results in an increase in the level of the focusing signal. In response to the motor control signal, the motor driving circuit 9 drives a motor 10 so that it rotates in a direction and at a speed indicated by the motor control signal. That is, a closed loop is formed in the circuit so that the focusing lens 1 is moved to a location at which the level of the focusing signal has a peak value. In the above technique of automatic focus control, the peak of the focusing signal is searched for, and thus this technique is called hill-climbing control.
In the above hill-climbing control, however, when the contrast of the object within the distance measurement frame is rather low, if there is an object having a high contrast outside the distance measurement frame, there is a possibility that it may become unable to correctly control the focusing. The above problem will be described in greater detail below with reference to FIGS. 14 to 16.
FIG. 14 illustrates three different focusing states: a best focus state; modestly defocused states 2; and a highly defocused state. In FIG. 14, the object A present outside the distance measurement frame has a higher contrast than the object B present in the distance measurement frame. In the best focus state shown in FIG. 14(1), the object A is completely outside the distance measurement frame, and the object B is completely inside the distance measurement frame. In the modestly defocused state shown in FIG. 14(2), the outline of the object A is blurred, and a part of the blurred image (a circle of confusion) comes in contact with the periphery of the distance measurement frame. In the greatly defocused state shown in FIG. 14(3), a part of the blurred image of the object A comes into the distance measurement frame.
If the objects A and B are equally apart from the image pick-up apparatus, the focusing signals associated with the objects A and B change with the position of the focusing lens as shown in FIG. 15. In this case, the focusing signal within the distance measurement frame changes in such a fashion as shown in FIG. 16.
Since the focusing signal is given for a portion detected as having the highest contrast within the distance measurement frame, the edge of the object A is detected in the greatly defocused state (3) or in the modestly defocused state (2). However, the focusing signal in connection with the object A decreases abruptly as the focusing state goes to a better state from the modestly defocused state (2) and thus the edge of the object A goes to the outside of the distance measurement frame. Finally, the edge of the object B is detected, and thus the focusing signal comes to be given for the object B. As a result, the focusing signal has a peak when the blurred edge of the object A located outside the distance measurement frame goes out of the distance measurement frame. This means that the focusing signal has a peak in the modestly defocused state (2), and the position corresponding to that peak is incorrectly regarded as a best focus position. Therefore, the true best focus position cannot be reached.
As described above, in the conventional automatic focus control technique in which the focus control operation is performed by searching for a peak in the focusing signal, if the contrast of an object within the distance measurement frame is rather low and if there is an object having a high contrast outside the distance measurement frame, a blurred edge portion of an object located outside the distance measurement frame can partially enter the distance measurement frame as a result of expansion of the image in a defocused state, and thus it becomes impossible to reach a correct focused state.
In view of the above problems, it is an object of the present invention to provide a method and apparatus for automatically controlling the focus by which a correct focused state can be obtained even when an object having a high contrast is present adjacent to the distance measurement frame.